2x Multiplus II 10KW in parallel single phase external transfer switch

So I've been scratching my head over something similar with my cabin setup and wondered if anyone's gone down this road.

I'm looking at running two Multiplus II 5kVA units in parallel (single phase) to cover the cabin and potentially the garden office as well. The question that's keeping me up at night is around the AC output and grid connection arrangement when using an external transfer switch rather than relying purely on the built-in switching.

Specifically — is it acceptable practice to combine the AC outputs from both units onto a single busbar alongside the grid input feed? I know Victron's documentation talks about this in the context of three-phase setups with larger inverters, but the guidance feels a bit vague when you're doing single-phase parallel with an external transfer switch thrown into the mix.

My concerns are:

• Are there any timing/sync issues if both Multiplus units aren't switching simultaneously?
• Does the external transfer switch need to see the combined output, or just the master?
• Any UK-specific regs (Part P, DNO notifications etc.) I should be aware of before wiring this up?

I've had a dig through VictronConnect and the Victron Community forums but the answers seem geared toward much bigger commercial installs or three-phase setups on the continent.

Anyone running a similar parallel single-phase arrangement in the UK — particularly off-grid with occasional grid top-up? Would love to know how you've physically wired it and whether your DNO had anything to say about it.

@Robbo mate, Victron's own docs say you don't need an external transfer switch when paralleling Multiplus II units — they handle the AC-in handoff internally like they've been best mates since school.

@BC_Boats is right that the internal transfer is handled natively, but worth flagging — when you're paralleling two units you absolutely must use a proper VE.Bus connection between them and let one act as master. The synchronisation happens automatically once that's configured in VEConfigure.

What caught me out on my narrowboat build was the AC input current limit — with two units paralleled it doesn't simply double. Each unit manages its own input, so if you're on shore power with a 16A supply, configure accordingly or you'll trip the hook-up constantly.

Also make sure your DC cabling to both units is identical length and cross-section — even small imbalances cause one unit to work harder and you'll see it in the VRM data pretty quickly.

Fogstar drift cells paired with Multiplus II parallel setups handle the charge coordination beautifully in my experience.

@Boycie is cut off mid-sentence which is frustrating — finish your thought mate, that's clearly the important bit.

From my own garden office setup (single Multiplus II 5kVA), the internal transfer is genuinely fast enough that sensitive electronics don't blink. Paralleling should maintain that, but I'd still double-check your VEConfigure settings are mirrored identically across both units — even a minor mismatch in AC input voltage thresholds between the two can cause weird handoff behaviour when mains drops.

Also worth confirming: are you using a MK3-USB or GX device for the parallel comms cable? Seen people trip over the RJ45 CAT5 wiring between units being done wrong and then blaming the inverters.

What's your expected peak load? Two 5kVA units parallel should give you ~8kVA continuous usable — plenty for a cabin unless you're running something industrial.

Aye @GoldenSocket is right, @Boycie left us hanging there! Frustrating timing.

To add something useful while we wait — one thing worth considering with two Multiplus II 5kVAs in parallel is making sure your VE.Bus cabling between the units is properly equalised in length. Victron are quite specific about this and it's easy to overlook. Unequal cable lengths can cause load sharing issues between the units.

Also @Robbo, have you configured them through VEConfigure yet? When paralleling you'll need to set one as master and one as slave — it's straightforward enough but you must get that right before energising anything. The Victron documentation on parallel operation covers it well but shout if you need a hand finding it.